Flexible circuitry is used in a wide variety of applications to provide electrical conductor paths. For instance, flexible circuitry has been used in electrical switches, such as membrane switches.
Generally, a flexible circuit is formed by a sheet or layer of flexible film, such as polyester film, having an electrical circuit or conductor pattern on at least one side thereof. For instance, the electrical circuit may be an inked conductor pattern. Typically, the flexible circuit is adhered to some form of stiffener or more rigid backing substrate to provide support for the flexible circuit. In other words, to support the flexible circuit in use, such as in a switch, a supporting structure must be used to prevent the flexible component from bending or collapsing. Moreover, such flexible circuits must be electrically linked to an electrical device by a connector or heat seal connection.
An example of an electrical switch in which flexible circuitry is used is in the area of membrane switches. Such a switch often includes a generally planar stiffener or substrate to which a flexible circuit is adhered. One side of the flexible film of the circuit is adhered to the stiffener, and the opposite side has a circuit pattern, including an open circuit portion, printed thereon. In some instances, the stiffener may be provided with formed or bent spring beams to engage appropriate circuit traces on a printed circuit board. The flexible circuit has an area with circuit terminal portions on the bent spring beams for electrical connection to the traces on the printed circuit board. A top layer typically made of insulating material is provided over the flexible circuit and includes a contact area on a side of the layer in facing relation with the open circuit portion of the circuit pattern on the flexible circuit. Movement of the top layer toward the flexible circuit is effective to move the contact area and close the open circuit portion. A spacer layer may be provided between the top layer and the flexible circuit, except between the contact area and the open circuit portion, to normally maintain the switch in open condition. In such a structure, the top layer does not have to be made of insulative material.
"Referring to the drawings in greater detail, and first to FIGS. 1-4, a membrane switch, generally designated 12, is shown in accordance with the prior art. The switch includes four major components, namely: a stiffener 14; a flexible circuit, generally designated 16; a spacer layer 18; and an actuator layer 20. These components are assembled together in sort of a lamination as seen best in FIGS. 3 and 4.
More particularly, stiffener 14 of prior art switch 12 is a simple planar metal sheet of a rectangular configuration having a plurality of spring beams 22 bent or formed out of the sheet. As seen in FIGS. 3 and 4, the distal ends of spring beams 22 have feet portions 22a for biasing toward a printed circuit board (not shown) in the direction of arrows "A".
Flexible circuit 16 of prior art switch 12 includes a flexible film 24, such as of polyester material. The film is rectangularly shaped to the same dimensions as stiffener 14, whereby a back side 24a of the film can be adhered, as by an appropriate adhesive, to the front face of the stiffener. A given pattern of conductors 26 is printed or inked onto the front face 24b of flexible film 24, i.e. on the side of the film opposite to side 24a which is adhered to stiffener 14. The printed conductors define a circuit pattern having a pair of open circuit portions 26a. The printed conductors also include terminal portions 26b extending to an edge of a tongue portion 24c of film 24. As seen best in FIGS. 3 and 4, tongue portion 24c is wrapped around the outside of spring beams 22 of stiffener 14 so that terminal portions 26b (FIG. 1) of the circuitry is in position to engage the circuit traces on the printed circuit board, as at arrows "A".
Spacer layer 18 of prior art switch 12 is adhered, as by an appropriate adhesive, to the front face of flexible circuit 16. The spacer layer substantially covers the printed circuit pattern 24 on the flexible circuit except for holes 28 in the spacer layer which expose open circuit portions 26a of the printed circuitry. Spacer layer 18 is rectangularly shaped and of the same dimensions as stiffener 14 and flexible circuit 16, except that the lower edge of the spacer layer is provided with a cutout 30 to accommodate spring beams 22 of the stiffener.
Actuator layer 20 of prior art switch 12 is adhered, as by an appropriate adhesive, to the front face of spacer layer 18. Again, the actuator layer is generally rectangular and of the same dimensions as the spacer layer, the flexible circuit and the stiffener, except for a cutout 32 aligned with cutout 30 in the spacer layer to accommodate spring beams 22. Actuator layer 20 may be fabricated of an insulative material such as polyester film. However, if the space layer 18 is made of an insulative material, the actuator layer 20 may be made of a conductive material. The actuator layer 20 includes formed or embossed "domes" 34, which project outwardly of the plane of the actuator layer 20. As seen in FIG. 4, a contact area in the form of a conductive ink pad 36 is printed to the concave inside of each dome 34. Each dome and its respective conductive pad 36 is aligned with a respective one of the holes 28 in spacer layer 18 which, in turn, is aligned with a respective one of the open circuit portions 26a of printed circuitry 26 on flexible circuit 16. It should be noted that the actuator layer 20 and spacer layer 18 can be replaced by metal domes, silicone membranes or any other device which functions to connect the open circuit portions 26a.
In the normally open condition of prior art switch 12, domes 34 and conductive pads 36 are spaced away from open circuit portions 26a as best seen in FIG. 4. When it is desired to close one or both of the switches, pressure is applied to one or both of the domes in the direction of arrows "B" (FIGS. 3 and 4) which moves conductive pad(s) 36 into engagement with open circuit portion(s) 26a to close the circuit therethrough."
One of the problems with switches or other electrical products using flexible circuitry in a structural combination generally as described above, is that the flexible circuit has a tendency to become delaminated from the stiffener or rigid backing substrate. Additionally, the inked circuit pattern tends to rub off the flexible circuit. This is particularly true when the stiffener has three-dimensional or formed portions about which the flexible circuitry conforms, such as the bent spring beams described above. Moreover, tolerances associated with assembling the amorphous flexible circuit to the stiffener can be very large because of the imprecision inherent in such an assembly. The present invention is directed to solving these problems in such items as electrical switches by eliminating the use of flexible circuitry yet providing similar advantages.